1. Technical Field
The present invention relates to a thermopile infrared detecting element and a method of manufacturing the same.
2. Background Art
Infrared radiation is emitted from the surface of a human body in an amount according to the body temperature, and ears have the normal temperature like the axillae. Therefore, clinical ear thermometers (temperature measuring devices) for measuring the body temperatures by detecting infrared radiation emitted in the ear cavities are known. In the clinical ear thermometers, the body temperatures are determined based on output from infrared detecting elements for converting received infrared radiation into electrical signals. Although various types of elements such as a pyroelectric type, a thermopile type, and the like are generally known as infrared detecting elements, thermopile-type infrared detecting elements having the merit of permitting mass production using a semiconductor manufacturing process and miniaturization are used as thermosensitive elements of clinical thermometers. Hereafter, the thermopile-type infrared detecting elements are simply referred to as “infrared detecting elements” unless otherwise stated.
FIG. 26 schematically shows an infrared sensor 100 provided with a thermopile infrared detecting element 110. The infrared sensor 100 comprises the infrared detecting element 110 provided with a thermopile 12, and a thermistor 120, the infrared detecting element 110 and the thermistor 120 being mounted on a package substrate 130, and contained in a case 140 to form a unit as a whole. In the infrared sensor 100, the thermistor 120 is used for determining the reference temperature of the thermopile 12 formed on the infrared detecting element 110, i.e., it is used for determining the temperature of cold junctions.
The infrared detecting element 110 comprises a membrane portion 116 comprising a membrane formed by etching a silicon substrate 2 to hollow the central portion 10 of the lower portion or the back side, and a thick wall portion 117 remaining unetched after etching of the silicon substrate 2. Namely, the infrared detecting element 110 has a structure in which the central portion of the lower side of silicon substrate 2 is made hollow to form a thin film (membrane) at the top. Also, gold black is deposited on the central portion 10 of the silicon substrate 2, i.e., on the membrane portion 116, by a sputtering deposition method or the like to form an infrared absorber 11 absorbing infrared radiation.
This infrared absorber 11 absorbs infrared radiation to cause a temperature change so that the temperature change is detected by a plurality of thermocouples 14 provided on the four sides of the infrared absorber 11. The hot junction 17 of each of the thermocouples 14 is disposed near the infrared absorber 11 of the membrane portion 116, and the cold junction 18 of each of the thermocouples 14 is disposed above the thick wall portion 117 in the peripheral portion 9 of the silicon substrate 2. The thermocouples 14 are connected in series to form the thermopile 12.
In a temperature measuring device using the infrared sensor 100, an electromotive force corresponding to a temperature change produced between the hot junctions 17 and the cold junctions 18 of the thermopile 12 is first detected, and a temperature difference between the hot junctions 17 and the cold junctions 18 is calculated based on the output voltage. Then, the temperature of the cold junctions 18 is calculated based on output of the thermistor 120, and the temperature difference is corrected by the temperature of the cold junctions 18 to determine a body temperature. However, the thermistor 120 used for determining the temperature of the cold junctions 18 of the thermopile 12 is disposed in a side portion at a certain distance from the infrared detecting element 110, and thus the peripheral temperature of the infrared detecting element 110 is simply detected. Therefore, it cannot be said that the accurate temperature of the cold junctions is detected. Thus, a large error possibly occurs in the body temperature determined as described above. Particularly, a clinical thermometer is required to have an accuracy of ±0.1° C. in the temperature range of 37° C. to 39° C., and required to perform temperature measurement with high accuracy, thereby causing the need to decrease such an error as much as possible.
The infrared sensor 100 is also required to have a space for arranging the thermistor 120, and thus has a problem in which the element itself cannot be made compact. Therefore, even when the thermistor 120 can be arranged in contact with the infrared sensor 100 to improve the accuracy of the cold junction temperature detected by the thermistor 120, the infrared detecting element cannot be readily effectively arranged in a compact form.
Accordingly, the applicant proposes an infrared detecting element comprising a PN junction, for example, a diode, incorporated into the silicon substrate 2, wherein the cold junction temperature is detected by using the fact that a forward voltage drop of the diode approximately linearly changes depending upon temperature.
This infrared detecting element can accurately detect the cold junction temperature, and can be arranged in a very compact form.
FIG. 27 is a cross-sectional view showing a portion of the infrared detecting element. The infrared detecting element 150 shown in FIG. 27 comprises a diode D formed in the peripheral portion 9 of the silicon substrate 2 so that the temperature of the cold junctions 18 of the thermopile 12 is detected by the diode D. The diode D comprises P+ anode region DP and N+ cathode region DN which are formed by ion implantation into a region isolated by forming a field oxide film (LOCOS) 151 on the surface of the silicon substrate 2. Namely, the diode D can be formed by a semiconductor process comprising forming the field oxide film 151 on the silicon substrate 2, and then patterning the oxide film 151 in the peripheral portion 9 of the silicon substrate 2 to form the regions DP and DN on the surface of the silicon substrate 2. In a thermopile-type infrared detecting element, after the diode D comprising the regions DP and DN is formed, a polysilicon electrical conductor 16 and an aluminum electrical conductor 15, which constitute the thermocouples 14, an oxide film 152, a surface protecting film 153, and an infrared absorber 11 are also formed by using the semiconductor manufacturing process. Therefore, the thermopile-type infrared detecting element provided with the diode can be simply formed by a series of steps of the semiconductor manufacturing technique, decreasing manufacturing cost.
In the infrared detecting element 150, the diode D can be disposed near the cold junctions 18 of the thermopile 12 to permit the accurate detection of the temperature of the cold junctions 18. Also, the space for arranging the thermistor 120 can be reduced to realize an infrared detecting element arranged in a compact unit including the function to measure a reference temperature.
On the other hand, there is always demand for an improvement in the measurement accuracy of the thermopile 12.
In the infrared detecting element 150, the field oxide film 151 has a very low rate of etching with an etchant for etching the silicon substrate, for example, xenon fluoride, potassium hydroxide, or the like, and thus functions as a stopper in etching the central portion 10 of the silicon substrate 2 from the lower side or the back thereof. However, even when the field oxide film 151 has a low etching rate, it is etched, and thus the thickness of the field oxide film 151 must be set to about 5000 to 7000 Å in order to impart the sufficient function as the stopper to the field oxide film 151. On the other hand, the field oxide film 151 comprises a film having internal stress in the compression direction, and thus has the property of being convexly curved due to expansion. The internal stress is as large as 2 to 3×109 dyne/cm2 (1 dyne/cm2=0.1 Pa). Therefore, when the temperature rises, the field oxide film 151 comprising such a thick film as described above is possibly bent due to the internal stress in the compression direction or broken due to deformation.
Furthermore, bending of the field oxide film 151 causes a change in the shapes of the aluminum electrical conductor 15 and the polysilicon electrical conductor 16 of each of the thermocouples 14 formed on the field oxide film 151, and thus the resistances of these electrical conductors are increased compared with the original resistances to cause the occurrence of a useless voltage drop. This influences the measurement accuracy of the temperature difference between the hot junctions 17 and the cold junctions 18, which is measured by the thermopile 12. Therefore, the infrared detecting element 150 has high accuracy because the cold junction temperature can be accurately detected by the diode D. However, in temperature measurement with higher accuracy, measurement error of the thermopile 12 due to deformation of the field oxide film 151 cannot possibly be neglected. Also, the degree of bending of the field oxide film 151 varies with various factors such as variations in the manufacturing process, atmospheric pressure, environmental temperature, etc., and it is thus very difficult to remove the influence of deformation of the field oxide film 151 by correcting the temperature, which is detected by the thermopile 12, using an appropriate factor.
Accordingly, an object of the present invention is to provide an infrared detecting element and a method of manufacturing the same capable of accurately detecting a cold junction temperature by using a PN junction, and capable of removing measurement error of a thermopile due to deformation of a membrane or the like to permit temperature detection with higher accuracy. Another object of the present invention is to provide a temperature measuring device capable of accurately measuring temperature by using the infrared detecting element without being influenced by changes in the measurement environment.